直接上原始碼，這裡面的signal比較簡單，主要學會sigaction的用法

  struct sigaction sigact;
  sigact.sa_handler = sig_handler;
  sigact.sa_flags = 0;
  sigemptyset(&sigact.sa_mask);
  sigaction(SIGUSR1, &sigact, NULL);
  sigaction(SIGUSR2, &sigact, NULL);
  sigaction(SIGHUP, &sigact, NULL);
  sigaction(SIGTERM, &sigact, NULL);
  sigaction(SIGALRM, &sigact, NULL);
  sigaction(SIGCHLD, &sigact, NULL);

  /* ignore SIGPIPE */
  sigact.sa_handler = SIG_IGN;
  sigaction(SIGPIPE, &sigact, NULL);

 

直接進入到訊號handler函式

static void sig_handler(int sig)
{
  if (pid == 0)
    {
      /* ignore anything other than TERM during startup
	 and in helper proc. (helper ignore TERM too) */
      if (sig == SIGTERM)
	exit(EC_MISC);
    }
  else if (pid != getpid())
    {
      /* alarm is used to kill TCP children after a fixed time. */
      if (sig == SIGALRM)
	_exit(0);
    }
  else
    {
      /* master process */
      int event, errsave = errno;
      
      if (sig == SIGHUP)
	event = EVENT_RELOAD;
      else if (sig == SIGCHLD)
	event = EVENT_CHILD;
      else if (sig == SIGALRM)
	event = EVENT_ALARM;
      else if (sig == SIGTERM)
	event = EVENT_TERM;
      else if (sig == SIGUSR1)
	event = EVENT_DUMP;
      else if (sig == SIGUSR2)
	event = EVENT_REOPEN;
      else
	return;

      send_event(pipewrite, event, 0); 
      errno = errsave;
    }
}
 

收到訊號之後，往pipe寫資料，

繼續看

void send_event(int fd, int event, int data)
{
  struct event_desc ev;
  
  ev.event = event;
  ev.data = data;
  
  /* error pipe, debug mode. */
  if (fd == -1)
    fatal_event(&ev);
  else
    /* pipe is non-blocking and struct event_desc is smaller than
       PIPE_BUF, so this either fails or writes everything */
    while (write(fd, &ev, sizeof(ev)) == -1 && errno == EINTR);
}
 

 if (FD_ISSET(piperead, &rset))
	async_event(piperead, now);

static void async_event(int pipe, time_t now)
{
  pid_t p;
  struct event_desc ev;
  int i;

  if (read_write(pipe, (unsigned char *)&ev, sizeof(ev), 1))
    switch (ev.event)
      {
      case EVENT_RELOAD:
	clear_cache_and_reload(now);
	if (daemon->port != 0 && daemon->resolv_files && (daemon->options & OPT_NO_POLL))
	  {
	    reload_servers(daemon->resolv_files->name);
	    check_servers();
	  }
#ifdef HAVE_DHCP
	rerun_scripts();
#endif
	break;
	
      case EVENT_DUMP:
	if (daemon->port != 0)
	  dump_cache(now);
	break;
	
      case EVENT_ALARM:
#ifdef HAVE_DHCP
	if (daemon->dhcp)
	  {
	    lease_prune(NULL, now);
	    lease_update_file(now);
	  }
#endif
	break;
		
      case EVENT_CHILD:
	/* See Stevens 5.10 */
	while ((p = waitpid(-1, NULL, WNOHANG)) != 0)
	  if (p == -1)
	    {
	      if (errno != EINTR)
		break;
	    }      
	  else 
	    for (i = 0 ; i < MAX_PROCS; i++)
	      if (daemon->tcp_pids[i] == p)
		daemon->tcp_pids[i] = 0;
	break;
	
      case EVENT_KILLED:
	my_syslog(LOG_WARNING, _("child process killed by signal %d"), ev.data);
	break;

      case EVENT_EXITED:
	my_syslog(LOG_WARNING, _("child process exited with status %d"), ev.data);
	break;

      case EVENT_EXEC_ERR:
	my_syslog(LOG_ERR, _("failed to execute %s: %s"), 
		  daemon->lease_change_command, strerror(ev.data));
	break;

	/* necessary for fatal errors in helper */
      case EVENT_HUSER_ERR:
      case EVENT_DIE:
	fatal_event(&ev);
	break;

      case EVENT_REOPEN:
	/* Note: this may leave TCP-handling processes with the old file still open.
	   Since any such process will die in CHILD_LIFETIME or probably much sooner,
	   we leave them logging to the old file. */
	if (daemon->log_file != NULL)
	  log_reopen(daemon->log_file);
	break;
	
      case EVENT_TERM:
	/* Knock all our children on the head. */
	for (i = 0; i < MAX_PROCS; i++)
	  if (daemon->tcp_pids[i] != 0)
	    kill(daemon->tcp_pids[i], SIGALRM);
	
#if defined(HAVE_DHCP) && defined(HAVE_SCRIPT)
	/* handle pending lease transitions */
	if (daemon->helperfd != -1)
	  {
	    /* block in writes until all done */
	    if ((i = fcntl(daemon->helperfd, F_GETFL)) != -1)
	      fcntl(daemon->helperfd, F_SETFL, i & ~O_NONBLOCK); 
	    do {
	      helper_write();
	    } while (!helper_buf_empty() || do_script_run(now));
	    close(daemon->helperfd);
	  }
#endif
	
	if (daemon->lease_stream)
	  fclose(daemon->lease_stream);

	if (daemon->runfile)
	  unlink(daemon->runfile);
	
	my_syslog(LOG_INFO, _("exiting on receipt of SIGTERM"));
	flush_log();
	exit(EC_GOOD);
      }
}